The systemic hematological effects of low-dose intraarterial streptokinase were studied in 12 patients with angiographic evidence of thrombosis. Streptokinase was administered at 5,000 U/hr; this rate was increased in the absence of clinical response or a systemic effect, which was defined as a thrombin time greater than twice that of the control time and/or a fibrinogen concentration less than 30% that of the control concentration. Eight patients experienced a systemic effect with final infusion rates of 7,500 to 20,000 U/hr over 20 to 72 hours, while 4 patients had no evidence of a systemic effect at 5,000 to 10,000 U/hr over 18 to 138 hours. A systemic effect could not be predicted based on infusion rates or durations, and the presence of a systemic effect was not predictive of hemorrhagic complications or thrombolytic effect in our small series. These results suggest that a systemic effect is frequently encountered with this form of therapy and is of uncertain clinical relevance.